The invention relates to an air-conditioning system for a motor vehicle.
Particularly in low-consumption vehicles, there is often a lack of heating power available for heating purposes when driving off on cold days, and consequently it is necessary to employ additional heating measures. To reduce the energy consumption of motor vehicles and to save energy, heat pump systems are used in air-conditioning systems of low-consumption vehicles. Many heat pumps use the ambient air as heat source. Since in this case ambient heat is rendered usable for heating purposes, systems of this type may have an advantageous energy consumption. As the outside temperatures drop, so does the supply of heat to a heat pump of this type. This runs contrary to the additional heating demand, which rises as the ambient temperature drops, and consequently heat pump systems of this type soon reach the limits of their use at which they are no longer able to cover the demand for power. Further problems, such as for example frosting or icing of the heat exchanger which extracts heat from the ambient air, require additional measures to be used.
Many heat pumps additionally use the evaporator which cools the passenger compartment feed air in cooling mode as a heater which heats the passenger compartment feed air in heating mode. However, this has the drawback that condensation precipitates at the evaporator/heater during cooling and dehumidification. If this wet evaporator is used for heating, flash fogging of the windows occurs, which must be avoided at all costs for safety reasons. This type of sequence, in which heating and cooling closely follow one another in terms of time often occurs in the transitional seasons of the year in spring and fall.
Only the addition of a further heat exchanger in the feed air stream which is used only for heating purposes, whereas the evaporator is used exclusively for cooling and dehumidification purposes, provides a reliable remedy to this problem without any limitation, as described, for example, in DE 39 07 201.
If the engine cooling water which is heated by the engine waste heat is used as heat source, the system is not so dependent on the ambient conditions, since the engine cooling water heats up quickly over the course of time and consequently a productive and powerful heat source is provided for the heat pump. This is disclosed, for example, in DE 36 35 353.
In DE 196 44 583 A1, it is on the one hand proposed that the engine waste heat be used as heat source (for example including the engine cooling water), and on the other hand that the heat be transferred to the passenger compartment feed air by means of a dedicated heat exchanger. A system of this type includes at least four heat exchangers:    1. a heat exchanger which in cooling mode dissipates the waste heat from the cycle process to the environment (in conventional systems with a condensing working medium, this is referred to as a condenser, whereas in CO2 systems operated under supercritical conditions it is referred to as a gas cooler),    2. an evaporator which in cooling mode cools and dehumidifies the passenger compartment feed air,    3. a heat exchanger which heats the passenger compartment feed air in heating mode (heater), and    4. a heat exchanger which in heating mode takes up engine waste heat (heat pump evaporator).
In the case of the supercritical CO2 process, there is also an inner heat exchanger, which exchanges heat between the high-pressure side and low-pressure side, in order to increase the performance and efficiency.
Therefore, in both heating mode and cooling mode, two heat exchangers are operating, while the other two are inoperative. This can give rise to very different demands for refrigerant in the two operating states. In the heating mode, the refrigerant in the inoperative components which are in communication with the cold ambient air can completely condense, and consequently the highest demand for refrigerant is during heating mode. Completely blocking off the inoperative branches is unable to reliably suppress the displacement of refrigerant, since considerable quantities of refrigerant can still enter these inoperative parts of the refrigerant cycle through slight leaks at the valves or as a result of unfavorable conditions when switching over between the modes. This displacement of refrigerant can in principle be effected by holding in stock a sufficient quantity of refrigerant and buffering this quantity of refrigerant by means of a sufficiently large collector. However, this should be avoided for reasons of the space taken up and also for safety reasons in particular when using the toxic refrigerant carbon dioxide.
It is an object of the invention to avoid the abovementioned drawbacks and to provide an improved refrigerant circuit.